MSG-treated Hamsters Research Articles

Neonatal monosodium glutamate treatment counteracts circadian arrhythmicity induced by phase shifts of the light–dark cycle in female and male Siberian hamsters

Studies of rats and voles suggest that distinct pathways emanating from the anterior hypothalamic-retrochiasmatic area and the mediobasal hypothalamic arcuate nucleus independently generate ultradian rhythms (URs) in hormone secretion and behavior. We evaluated the hypothesis that destruction of arcuate nucleus (ARC) neurons, in concert with dampening of suprachiasmatic nucleus (SCN) circadian rhythmicity, would compromize the generation of ultradian rhythms (URs) of locomotor activity. Siberian hamsters retain-->of both sexes treated neonatally with monosodium glutamate (MSG) that destroys ARC neurons were subjected in adulthood to a circadian disrupting phase-shift protocol (DPS) that produces SCN arrhythmia. MSG treatments induced hypogonadism and obesity, retain-->and markedly reduced the size of the optic chiasm and optic nerves. MSG-treated hamsters exhibited normal entrainment to the light–dark cycle, but MSG treatretain-->ment counteracted the circadian arrhythmicity induced by the DPS protocol: only 6% of retain-->MSG-treated hamsters exhibited circadian arrhythmia, whereas 50% of control hamsters were circadian disrupted. In MSG-treated hamsters that retained circadian rhythmicity after DPS treatment, quantitative parameters of URs appeared normal, but in the two MSG-treated hamsters that became circadian arrhythmic after DPS, both dark-phase and light-phase URs were abolished. Although preliminary, these data are consistent with reports in voles suggesting that the combined disruption of SCN and ARC function impairs the expression of behavioral URs. The data also suggest that light thresholds for entrainment of circadian rhythms may be lower than those required to disrupt circadian organization.

Acute brown adipose tissue temperature response to cold in monosodium glutamate-treated Siberian hamsters

Neonatal monosodium glutamate (MSG) administration increases adiposity, decreases energy expenditure and is associated with arcuate nucleus (Arc) destruction. Disrupted brown adipose tissue (BAT) thermogenesis underlies some of these effects, although, interscapular BAT temperature (TIBAT) has not been measured. Therefore, we tested the effects of neonatal MSG or vehicle administration in Siberian hamsters and, when they were adults, measured TIBAT during acute cold exposure. The Arc and its projection to the hypothalamic paraventricular nucleus (PVH) are both components of the CNS outflow circuits to IBAT, with the latter implicated in BAT thermogenesis that could be compromised by MSG treatment. Using a viral transneuronal tract tracer, pseudorabies virus (PRV), we also tested whether the components of these circuits were intact. As adults, MSG-treated hamsters had significantly increased body mass and some white fat pad masses, markedly reduced Arc Nissl and neuropeptide staining, and PVH neuropeptide fiber staining. Cold-exposed (18 h at 5 °C) MSG- and vehicle-treated hamsters initially maintained TIBAT, but the ability of the former waned after 2 h being significantly decreased by 18 h. PRV immunoreactive fibers/cells were not altered by neonatal MSG treatment despite substantial Arc and PVH destruction. MSG- and vehicle-treated hamsters given an exogenous norepinephrine challenge showed identical increases in the duration and peak of TIBAT. Thus, the inability of MSG-treated animals to sustain TIBAT in the cold is not due to any obvious MSG-induced deletions of central sympathetic outflow circuits to IBAT, but appears to be extrinsic to the tissue nevertheless.

Food deprivation-induced changes in body fat mobilization after neonatal monosodium glutamate treatment

The reversal of obesity is a difficult feat at best and is a growing problem as the obesity epidemic increases worldwide. Considerable focus has been made on the arcuate nucleus (Arc) in the control of body and lipid mass and food intake. To test the role of the Arc in body fat mobilization, we compared the effects of food deprivation on white adipose tissue (WAT) mass in adult Siberian hamsters by making exocytotic lesions of the Arc via neonatal subcutaneous injections of monosodium glutamate (MSG). MSG-treated hamsters had significantly increased body mass, total and individual WAT pad masses, and serum leptin concentrations compared with their vehicle-injected counterparts. MSG produced marked reductions in Arc Nissl staining, tyrosine hydroxylase-immunoreactive (ir) neurons, and neuropeptide Y (NPY)- and agouti-related protein (AgRP)-ir fibers compared with controls. MSG significantly decreased hypothalamic paraventricular nucleus (PVN) NPY- and AgRP fiber-ir compared with controls, likely because of Arc projections to this nucleus. MSG treatment also reduced area postrema (AP) tyrosine hydroxylase (TH)-ir fibers compared with controls. MSG treatment did not, however, block food deprivation-induced decreases in WAT pad mass compared with controls. Thus, despite considerable damage to the Arc and some of its projections to the PVN, as well as the AP, body fat was mobilized apparently normally, bringing into question the necessity of these structures for food deprivation-induced lipid mobilization. These data support recent evidence that chronically decerebrate rats, in which the forebrain is surgically isolated from the caudal brainstem, show normal food deprivation responses, including lipid mobilization.

Monosodium glutamate-induced arcuate nucleus damage affects both natural torpor and 2DG-induced torpor-like hypothermia in Siberian hamsters

Siberian hamsters (Phodopus sungorus) have the ability to express daily torpor and decrease their body temperature to approximately 15 degrees C, providing a significant savings in energy expenditure. Daily torpor in hamsters is cued by winterlike photoperiods and occurs coincident with the annual nadirs in body fat reserves and chronic leptin concentrations. To better understand the neural mechanisms underlying torpor, Siberian hamster pups were postnatally treated with saline or MSG to ablate arcuate nucleus neurons that likely possess leptin receptors. Body temperature was studied telemetrically in cold-acclimated (10 degrees C) male and female hamsters moved to a winterlike photoperiod (10:14-h light-dark cycle) (experiments 1 and 2) or that remained in a summerlike photoperiod (14:10-h light-dark cycle) (experiment 3). In experiment 1, even though other photoperiodic responses persisted, MSG-induced arcuate nucleus ablations prevented the photoperiod-dependent torpor observed in saline-treated Siberian hamsters. MSG-treated hamsters tended to possess greater fat reserves. To determine whether reductions in body fat would increase frequency of photoperiod-induced torpor after MSG treatment, hamsters underwent 2 wk of food restriction (70% of ad libitum) in experiment 2. Although food restriction did increase the frequency of torpor in both MSG- and saline-treated hamsters, it failed to normalize the proportion of MSG-treated hamsters undergoing photoperiod-dependent torpor. In experiment 3, postnatal MSG treatments reduced the proportion of hamsters entering 2DG-induced torpor-like hypothermia by approximately 50% compared with saline-treated hamsters (38 vs. 72%). In those MSG-treated hamsters that did become hypothermic, their minimum temperature during hypothermia was significantly greater than comparable saline-treated hamsters. We conclude that 1) arcuate nucleus mechanisms mediate photoperiod-induced torpor, 2) food-restriction-induced torpor may also be reduced by MSG treatments, and 3) arcuate nucleus neurons make an important, albeit partial, contribution to 2DG-induced torpor-like hypothermia.

Seasonal Neuroendocrine Rhythms in the Male Siberian Hamster Persist After Monosodium Glutamate‐Induced Lesions of the Arcuate Nucleus in the Neonatal Period

The aim of these experiments was to examine the role of the arcuate nucleus in the control of seasonal cycles of body weight, feed intake, moulting and reproduction in the Siberian hamster. The arcuate nucleus has previously been implicated as a central site where systemic feedback signals (e.g. leptin) might act to regulate feed intake and body weight, so it was predicted that hamsters with lesions of this structure would be unable to display the inhibitory effects of short days on these parameters. In the first series of studies, lesions that destroyed approximately 80% of the cells in the arcuate nucleus were produced by treating hamsters neonatally with monosodium glutamate (MSG; 4 mg/g body weight sc), and vehicle- and MSG-treated males were raised from birth in long days (LD) or short days (SD). In hamsters raised in LD, the initial gain in body weight and testicular growth were significantly reduced by MSG treatment, however, growth rate and testis weight were still significantly greater than in vehicle- or MSG-treated hamsters raised in SD. In the second study, hamsters treated neonatally with vehicle or MSG were raised in LD for 8 weeks and, subsequently, approximately half in each group were transferred to SD for 18 weeks. As expected, vehicle-treated hamsters showed a characteristic decline in body weight when exposed to SD, while those remaining in LD continued to increase body weight. Feed intake decreased in parallel with the decline in body weight in SD, a complete moult to the white winter pelage occurred by 16 weeks in SD, and testicular regression occurred. Responses to SD also occurred in the MSG-treated hamsters: body weight decreased in SD but increased in their lesioned litter mates remaining in LD, and feed intake paralleled body weight changes in these groups. The moult to winter pelage was significantly retarded in MSG-treated hamsters transferred to SD. The testes were completely regressed in sham- and MSG-treated hamsters exposed to SD, whereas testes weights in MSG-treated hamsters maintained in LD were intermediate between those in vehicle-treated hamsters in SD and LD. Thus, despite initial effects on growth, the MSG-treated hamsters bearing substantial lesions of the arcuate nucleus were able to show appropriate responses to photoperiod, although not always of the same magnitude as the unlesioned controls. We conclude that feedback mechanisms operating via the arcuate nucleus are not the major regulators of seasonal cycles of body weight, feed intake, pelage and reproduction.

Retinal ganglion cells expressing the FOS protein after light stimulation in the Syrian hamster are relatively insensitive to neonatal treatment with monosodium glutamate

In nocturnal rodents, the c-fos gene is directly involved in the light mechanism of resetting of the suprachiasmatic nucleus (circadian clock). Light also induces c-fos expression in the retinal ganglion cell layer (GCL), but no attempt has been made to study the retinal responses to the phase-shifting effects of light. The expression of the Fos protein in each of the two populations of the GCL (displaced amacrine cells [DACs] and ganglion cells [GCs]) was analyzed in hamsters after light stimulation delivered early (circadian time [CT13]) and in the middle (CT18) of the subjective night. To evaluate as accurately as possible the number of GCs able to phase shift the locomotor activity rhythm (LAR), neonatal hamsters treated with monosodium glutamate (MSG) were also used, an in vivo model which displays retinal degeneration and LAR normally entrained by light. In nontreated hamsters, the number of Fos-immunoreactive (Fos-ir+) nuclei in the GCL was significantly higher at CT18 than at CT13. In MSG-treated hamsters, the number of Fos-ir+ nuclei was the same at both CTs and nonsignificantly different as those of nontreated hamsters at CT13. MSG treatment destroyed as many Fos-ir+ DACs as Fos-ir- DACs or Fos-ir+ GCs. Fos-ir+ GCs were less sensitive to neurotoxic than other GCs, as only 37% of them were destroyed by treatment versus 92% for Fos-ir- GCs. At CT18, a maximum of 3,500 GCs expressed Fos protein in nontreated hamsters versus only 2,200 in MSG-treated hamsters. This minor subgroup was sufficiently potent to normally synchronize the circadian rhythms to the Light/dark cycle in treated hamsters. J. Comp. Neurol. 392:458–467, 1998. © 1998 Wiley-Liss, Inc.

Retinal ganglion cells expressing the FOS protein after light stimulation in the Syrian hamster are relatively insensitive to neonatal treatment with monosodium glutamate

In nocturnal rodents, the c-fos gene is directly involved in the light mechanism of resetting of the suprachiasmatic nucleus (circadian clock). Light also induces c-fos expression in the retinal ganglion cell layer (GCL), but no attempt has been made to study the retinal responses to the phase-shifting effects of light. The expression of the Fos protein in each of the two populations of the GCL (displaced amacrine cells [DACs] and ganglion cells [GCs]) was analyzed in hamsters after light stimulation delivered early (circadian time [CT13]) and in the middle (CT18) of the subjective night. To evaluate as accurately as possible the number of GCs able to phase shift the locomotor activity rhythm (LAR), neonatal hamsters treated with monosodium glutamate (MSG) were also used, an in vivo model which displays retinal degeneration and LAR normally entrained by light. In nontreated hamsters, the number of Fos-immunoreactive (Fos-ir+) nuclei in the GCL was significantly higher at CT18 than at CT13. In MSG-treated hamsters, the number of Fos-ir+ nuclei was the same at both CTs and nonsignificantly different as those of nontreated hamsters at CT13. MSG treatment destroyed as many Fos-ir+ DACs as Fos-ir- DACs or Fos-ir+ GCs. Fos-ir+ GCs were less sensitive to neurotoxic than other GCs, as only 37% of them were destroyed by treatment versus 92% for Fos-ir- GCs. At CT18, a maximum of 3,500 GCs expressed Fos protein in nontreated hamsters versus only 2,200 in MSG-treated hamsters. This minor subgroup was sufficiently potent to normally synchronize the circadian rhythms to the Light/dark cycle in treated hamsters.

Neurotoxic effects of neonatal injections of monosodium L-glutamate (L-MSG) on the retinal ganglion cell layer of the golden hamster: anatomical and functional consequences on the circadian system.

In rodents, daily injection of neurotoxic monosodium L-glutamate (MSG) during the postnatal period induces retinal lesions, optic nerve degeneration with an alteration of visual pathway and an absence of the b-wave in the electroretinogram. Despite this damage, electrophysiological responses subsist in the lateral geniculate bodies and synchronization of circadian rhythms to the light/dark cycle can still occur. Using two formal properties of the circadian system (entrainment and phase-shift by light), we assessed the functionality of retinal projections to the circadian clock in MSG-treated hamsters. Displaced amacrine and ganglion cell populations were quantified and retinal terminals in the suprachiasmatic nuclei were estimated. Animals received daily doses of glutamate during the first ten days after birth according to two protocols. The two treatments similarly destroyed 56% of the overall population of the ganglion cell layer: 30% of displaced amacrine and 89% of ganglion cells. Surviving ganglion neurons (7,500 cells) were evenly distributed across the entire retina except in one area of high cell density located in the temporoventral quadrant. Retinal projections of the "image-forming" pathway were drastically reduced in the dorsal lateral geniculate bodies, less in their ventral part. The "nonimage-forming" pathway was also affected since the volume of labeled terminals in the suprachiasmatic nuclei was reduced by one-half to one-third. Nevertheless, treated hamsters exhibited a free-running locomotor activity rhythm after several months in constant darkness, could be entrained by the light/dark cycle and phase-shifted by light pulses. These results suggest that a damaged retinohypothalamic tract can still assume the photic entrainment of the circadian clock.

Alterations in Insulin and Glucagon Secretion by Monosodium Glutamate Lesions of the Hypothalamic Arcuate Nucleus

This study was performed to determine the effects of monosodium glutamate (MSG) induced lesions of the hypothalamic arcuate nucleus (ARC) on glucose tolerance and insulin and glucagon secretion in male golden hamsters. Eight day old hamsters were given a single s.c. injection of 5.8 mg/g BW MSG or hypertonic saline (controls). Studies were initiated when the hamsters were 3 months of age. At this age there were no body weight differences. Glucose (180 mg/100 g BW) was administered via stomach tube to 18 control and 18 MSG-treated hamsters. Animals were anesthetized with ether and a single blood sample from the portal vein was taken either before or at 30 or 60 min after glucose administration (n = 6/group). Glucose concentrations were similar in both groups at all time periods. Insulin concentrations in the MSG group were significantly (P less than 0.05) elevated in MSG-treated hamsters compared to controls at the 60 min time point. Glucose suppressed glucagon (P less than 0.05) in control but not in MSG-treated hamsters. The MSG group had significantly more glucagon (P less than 0.05) in portal vein blood at 30 min after glucose administration than did the control hamsters. Molar insulin/glucagon ratios did not differ between the 2 groups which likely accounts for the lack of differences in blood glucose levels. These results suggest a role for the ARC in regulating pancreatic function.

Effects of anterior hypothalamic deafferentation and arcuate nucleus lesions on the feedback actions of estradiol benzoate on luteinizing hormone and follicle-stimulating hormone secretion in the female hamster.

This investigation studied theeffectsof anterior hypothalamic deafferentation (EFC), a monosodium glutamate (MSG)-induced lesion of the arcuate nucleus (ARC), or a combination of these two procedures on the regulation of luteinizing hormone (LU) and follicle-stimulating hormone (FSH) secretion in the chronically ovariectomized hamster given estradiol benzoate (EB) or oil. MSG-treated or control animals were ovariectomized and subjected to EFC or sham deafferentation (SFC) at 2-3 months of age. Three weeks later, animals were given a single injection of 50 �ig of EB or oil and the inhibitory and stimulatory actions of EB were determined by measuring LH and FSH by radioimmunoassay (RIA) of plasma obtained at 0900 h (AM) and 1600 h (PM) 3 days later. The tonic secretion of LH in SFC hamsters given oil was significantly reduced by EFC or MSG-induced lesions of the ARC. EFC in combination with these lesions did not further reduce LH levels in these animals. EB suppressed AM levels of LU in SFC animals and induced a PM surge which was prevented by EFC. Although EB did not further suppress AM levels of LH in the MSGtreated animals, a PM elevation was apparent and was prevented by EFC. In contrast to oil-treated animals, MSG-treated hamsters with EFC and given EB had significantly reduced levels of LH in both the AM and PM when compared to all other treatment groups. In contrast to LH, plasma FSH levels were not altered by EFC in control animals given oil, whereas FSH was reduced in MSG-treated animals with or without EFC. EB did not suppress AM levels of FSH in either control or MSG-treated animals, but did induce a significant surge of FSH in the PM in both groups of animals that was prevented by EFC. These results suggest that in the ovariectomized hamster, the ARC and anterior hypothalamus (AH) act as a functional unit in regulating the tonic secretion of LH, whereas the AH has little role in regulating tonic FSH secretion. The results also confirm previous reports suggesting that the ARC is involved in the tonic regulation of FSH secretion and that the AH is necessary for the stimulatory effects of estrogen on gonadotropin secretion.

The effect of neonatally administered monosodium glutamate (MSG) on the development of retinofugal projections and the entrainment of circadian locomotor activity

The acute administration of monosodium- l -glutamate (MSG, 8 mg/g) to neonatal hamsters ( Mesocricetus auratus ) destroys the neurons of the inner retina, although some ganglion cells survive. Despite the severe optic nerve degeneration, MSG-treated hamsters synchronize their circadian locomotor activity to light:dark signals. The central projections of the surviving ganglion cells in these animals were determined using anterograde tracing techniques. The retinohypothalamic pathway responsible for the photic synchronization of the locomotor activity was unaffected by the neurotoxin, although the remaining visual pathways were markedly reduced. The results indicate a differential sensitivity of retinal ganglion cells to MSG. The resistance of the retinal ganglion cells that innervate the suprachiasmatic nucleus of the hypothalamus to neonatal MSG treatment is discussed in relationship to other findings. The results from this study and other physiological and morphological findings suggest that the retino-hypothalamic system is an anatomically independent component of the optic system.

Pituitary Responsiveness to LHRH Stimulation in Hamsters Treated Neonatally with Monosodium Glutamate

Previous studies have shown that neonatal administration of monosodium glutamate (MSG) results in a clearly defined lesion of the arcuate nucleus which disrupts mechanisms regulating normal FSH but not LH secretion in the adult female hamster. The present study was designed to investigate the effects of neonatally administered MSG or hypertonic saline (control) on the in vivo and in vitro responses to LHRH in adult animals. In order to evaluate these responses under a comparable hormonal background, all animals were ovariectomized at 2-3 months of age and given 50 micrograms of estradiol benzoate 3 weeks later. 24 h later, animals were anesthetized with sodium pentobarbital and a blood sample taken for baseline levels of gonadotropins; they were immediately given 5 or 25 ng LHRH intravenously, and LH and FSH responses to LHRH were determined by sampling at 15-min intervals over a 1-hour period. Pituitary content of LH and FSH was determined in a similar group of control and MSG-treated animals which did not receive the hypothalamic hormone. Basal LH secretion, the dose-response of LH to LHRH, and the pituitary concentration or content were similar in control and MSG-treated hamsters. In contrast to LH, basal FSH levels were significantly lower, the FSH responses to LHRH were greatly attenuated, and a dose response to LHRH was absent in MSG-treated animals when compared to control values. Pituitary FSH concentration and content did not differ between control and MSG-treated animals. However, when pituitaries were removed, hemidissected, preincubated for 1 h, and incubated in the presence or absence of 8.5 x 10(-9) or 8.5 x 10(-10) M LHRH for 3 consecutive 1-hour periods, no differences were noted between control and experimental animals regarding basal (no LHRH) or LHRH-stimulated release of LH and FSH. These results suggest that the decreased basal FSH levels and the attenuated FSH response to LHRH observed in vivo in MSG-treated animals may be due to an alteration in a neural or extraneural component which is involved in regulating the release of FSH by LHRH and which is evident in the absence of a functional arcuate nucleus and/or develops directly as the result of the neonatal administration of MSG.

The effects of a single injection of monosodium glutamate on the reproductive neuroendocrine axis of the female hamster.

Monosodium glutamate (MSG) was injected into the neonatal hamster to determine whether it was possible to produce a discrete lesion of the accurate nucleus to study the effects of this lesion on ovarian histology hypothalamic neurohistochemistry and plasma and pituitary levels of gonadotropins and to study the response of the hypothalamo-pituitary axis to ovariectomy. MSG was administered in a dose of 8 mg/gm body weight on Day 6 7 8 9 or 10 of the neonatal period and decapitated 6 or 12 hours later. The brains were prepared for histological study. A lesion of the arcuate nucleus o fteh hypothalamus was induced by injection of MSG on Days 6-8. When injection was performed on Day 9 or 10 the lesion was still prominent but less severe. Animals treated on Day 8 of the neonatal period were examined beginning on Day 30 for postovulatory vaginal discharge. Plasma luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured. Histochemical study of catecholamines was performed on a small piece of the diencephalon. Ovaries and uteri were removed and weighed. A 2nd group of hamsters treated on Day 8 with either saline or MSG were ovariectomized on Day 60. 7 days later the pituitary was removed and gonadotropin determinations made. Postovulatory vaginal discharges in MSG-treated hamsters were absent. Organ and body weights were significantly lower in the treated hamsters and histological evaluation of the ovaries suggested inhibition of follicular maturation. This may have been due to altered levels of LH and FSH found in the plasma of treated animals. Partial destruction of the tubero-infundibular catecholaminergic pathway was confirmed by histofluorescence technique.